The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
There are several different types of EPDs. In the partition type of EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148–1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. No. 5,961,804 and U.S. Pat. No. 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors. Yet another type of EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent, as disclosed in co-pending application U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000.
One problem associated with these EPDs is reverse bias. A reverse bias condition could occur when the bias voltage on a particular cell changes rapidly by a large increment or decrement and in conjunction with the presence of a stored charge resulting from the inherent capacitance of the materials and structures of the EPD. The reverse bias condition affects display quality by causing charged pigment particles in affected cells to migrate away from the position to which they have been driven. The following description along with FIGS. 1A, 1B, and 2 further illustrate this problem.
FIG. 1A shows a sectional view of an example EPD 100. The EPD 100 includes an upper dielectric layer 108, an upper electrode 112, an electrophoretic dispersion layer 102, a lower dielectric layer 110, and a lower electrode 114. The electrophoretic dispersion layer 102 contains a colored dielectric solvent 106 with a plurality of charged pigment particles 104. In one embodiment, the insulating material of the dielectric layers may comprise a non-conductive polymer. In another embodiment, the insulating material may include a microcup structure or a sealing and/or adhesive layer, as disclosed, for example, in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000, U.S. Ser. No. 10/222,297, filed on Aug. 16, 2002, U.S. Ser. No. 10/665,898, filed on Sep. 18, 2003 and U.S. Ser. No. 10/762,196, filed on Jan. 21, 2004.
FIG. 1B shows a simplified electrical equivalent circuit for EPD 100. Specifically, C1 and R1 represent the combined electrical capacitance and resistance of the upper dielectric layer 108 and the lower dielectric layer 110, respectively. C2 and R2 represent the electrical capacitance and resistance of the electrophoretic dispersion layer 102, respectively.
Suppose drive voltage generator 116 applies a square wave Vin to the upper electrode 112 and the lower electrode 114. The waveform of the voltage applied across the electrophoretic dispersion layer 102, Ved, has overshooting and undershooting portions as shown in FIG. 2. Particularly, when Vin drops to zero, Ved has a polarity opposite to the drive voltage Vin. This “undershooting”, representing the reverse bias condition, causes charged particles to migrate away from a position to which they have been driven and results in degradation of the image-retention characteristics of the EPD 100.
One solution to the aforementioned reverse bias problem has been disclosed by Hideyuki Kawai in application U.S. Ser. No. 10/224,543, filed Aug. 20, 2002, U.S. patent publication 20030067666, published Apr. 10, 2003. The solution attempts to address the undershooting phenomenon by applying an input biasing voltage that has a smooth waveform and meets certain time constant requirements. However, this solution is difficult and costly to implement. Therefore, there is a need for an improved driving scheme for an EPD.